Diabetes mellitus, including type 1 diabetes (T1D) is critical to the public health agenda because it affects 17.5 million people in the US and accounts for more than 174 billion in medical expenditures and serious preventable health complications that lower quality of life and increase morbidity and mortality. The incidence of pediatric T1D has been increasing world-wide and is an important driver of health care costs because it begins in childhood, lasts a lifetime, and has increasing prevalence. Adolescence is a time of heightened vulnerability to problematic glycemic control and health risk for individuals with T1D. The transition from mid to late adolescence is especially precarious based on increased risk for poor glycemic control and the potential for early development of complications involving damage to the kidney, eye, or cardiovascular system. Lifelong patterns of diabetes management may be established during late adolescence. Evidence that modifiable psychological and contextual influences increase the risk for problematic glycemic control during adolescence creates a unique opportunity for targeted preventive interventions focused on diabetes management. Prospective data are critical to design effective clinical interventions that identify adolescents with T1D who are most in need, at points of specific developmental vulnerability and target modifiable influences that disrupt their glycemic control and threaten their future health. We have established a multisite cohort (N=230) that was recruited at the cusp of adolescence (age 10) and followed to mid adolescence (age 14) with minimal attrition. Our findings have demonstrated significant prediction of clinically relevant patterns of deterioration in glycemic control. Our study will build on these promising findings by accomplishing the following aims: 1) to examine continuity and change in trajectories of glycemic control among mid to late adolescents with T1D (ages 15-19). 2) To test a novel, developmentally specific model of modifiable risk and resource factors on glycemic control among mid to late adolescents with T1D. 3) To identify clinically significant subgroups of trajectories of glycemic control and profils of risk factors. 4) To characterize the impact of glycemic control on objective measures of precursors of significant kidney, eye, and cardiovascular complications. Study methods involve a prospective four-year study of adolescents with T1D that will include a comprehensive measurement of psychological and contextual influences (risk and resource factors, treatment adherence, glycemic control, and biomarkers of significant health complications. Our findings will guide the development of model preventive programs that efficiently and effectively tailor clinical management for older adolescents with T1D by identifying the optimal timing and targets of preventive intervention to enhance glycemic control and reduce the risk for future diabetes-related health complications.